An image forming apparatus of the photosensitive latent image transfer type is composed so that it may form a visible image by pressing an image receiving sheet onto a portion of a photosensitive sheet at a pressure part therein, which is successively drawn from a photosensitive sheet roll and has latent images formed thereon at an irradiation part in the apparatus. However, there is a fixed distance between the irradiation part and the pressure part. Consequently, when a portion of a photosensitive sheet having a latent image formed thereon is carried to the pressure part, some portion of the photosensitive sheet between the pressure part and the irradiation part remains unexposed. Accordingly, if the next exposure is started in this condition, the unexposed portion is wastefully drawn without any treatment.
Therefore it is suggested that a distance corresponding to the unexposed portion of the photosensitive sheet should be wound back every time the formation of visible image at the pressure part is completed. Thus rewind devices for photosensitive sheet have already been developed, which may eliminate the above wastefulness by starting the next exposure at a portion immediately after the portion having been exposed in the preceding operation (hereinafter called exposure portion). As one of these rewind devices, for example, is known a disclosure (Application No. 63-101851) of Japanese Publication for Unexamined Patent.
More specifically, as shown in FIG. 7(a), in the image forming apparatuses having such a rewind device, at first an exposure is started at the irradiation part in the apparatus while a portion of a photosensitive sheet is being drawn from a photosensitive sheet roll(in the figure, an unexposure portion of a photosensitive sheet is shown with thick solid lines and an exposed portion thereof is shown with broken lines). Simultaneously as the exposure portion is carried to the pressure part in the apparatus by the drawing of the photosensitive sheet, an image receiving sheet is pressed onto the exposure portion and thereby a visible image is formed on the image receiving sheet. Thus as shown in FIG. 7(b), after the completion of the exposure at the irradiation part, a further drawing of the photosensitive sheet is continued until the rear edge of the exposure portion T reaches the pressure part.
In this way, when the formation of visible image on the image receiving sheet is completed, the drawing of the Photosensitive sheet is stopped for a moment as shown in FIG. 7(c). Next as shown in FIG. 7(d), the rewind device is actuated to wind back the photosensitive sheet. The winding back by the rewind device is performed until the rear edge of the exposure portion T reaches just before the irradiation part as shown in FIG. 7(e). Consequently, as shown in FIG. 7(f), by starting the next exposure after the completion of the winding back operation, a wasteful consumption of the photosensitive sheet is preventable since the unexposed portion between the newly exposed portion and the preceding one is reduced to a small amount.
However, in the method wherein a photosensitive sheet is simply wound back by reversely rotating a supply shaft of the photosensitive sheet roll, it is difficult to wind back the photosensitive sheet accurately just before the irradiation part. This is because the outside diameter of the photosensitive sheet roll varies depending on the remaining amount of the photosensitive sheet, and therefore the rewind amount per one rotation differs according to the outside diameter of the photosensitive sheet roll. Consequently, in the conventional photosensitive sheet rewind devices, a fixed rewind amount is predeterminately set in a control part since a distance between the irradiation part and the pressure part is constant. Thus the photosensitive sheet at the distance corresponding to the preset rewind amount is wound back by the control operation.
However, when the movement of the fixed distance like this is applied not only to the drawing operation for the photosensitive sheet but also to the winding back operation, the same control as in the drawing operation is necessary. As a result, the conventional rewind devices have problems in that they need a complicated composition and therefore their costs are very high due to the necessity of controlling a driving device for photosensitive sheet in both the drawing operation and the winding back operation.